The present invention relates to the art of oil pumping and more particularly to a shock absorber for use between the reciprocating member of an oil well pumping mechanism and a polish rod connected to the downwardly extending sucker rod string.
For many years in the oil industry some of the oil well pumping units have used a shock absorber between the polish rod and operating bridle of the reciprocating pumping mechanism. When employed, these shock absorbers were normally formed from elastomeric discs, such as shown in Case U.S. Pat. No. 4,176,714; Fix U.S. Pat. No. 4,354,397; and, Clayton U.S. Pat. No. 4,445,674. These patents are incorporated by reference as a disclosure of the background to which the present invention is directed. Such technology is well known and need not be repeated in this description of the invention. Recently it has been suggested to replace the elastomeric discs by a more reliable mechanism in the form of a coil spring as shown in Pelham U.S. Pat. No. 6,446,946, also incorporated by reference herein. By using a steel coil spring, long term deterioration and wear of the shock absorber itself is reduced. Control over movement of the polish rod is drastically improved. In Pelham U.S. Pat. No. 6,446,946 the coil spring is contained in a spring housing including two telescoping cylindrical cup-shaped members. The background and technology of using a single coil spring in a shock absorber as a replacement for the elastomeric discs is disclosed in this 2002 patent.
Elastomeric shock absorbers shown in Case U.S. Pat. No. 4,176,714 have been very successful in reducing the peak loads imposed by the sucker rod string during the pumping action. However, elastomeric discs tend to wear and deteriorate during long term operation, especially in adverse environments. Consequently, recently there has been an effort to replace the elastomeric discs with a mechanical device in the form of a steel coil spring, as shown in Pelham U.S. Pat. No. 6,446,946. This newly developed technology is now in its infancy and has presented practical difficulties, such as undue wear between the polish rod and the lower end plate of the spring housing. This wear is accentuated when a spring pilot is provided in the bottom cylindrical housing member to center the coil spring. Such pilot has a center opening which contacts or engages the reciprocally movable polish rod to cause wear on the polish rod due to transverse forces. This metal-to-metal wear reduces the effective life of the shock absorber and requires periodic inspection and maintenance to assure continued operation of the shock absorber in the well pumping mechanism. The pilot needs to have a given axial height and must be formed from a hard material, such as hardened steel. Thus, there is an extended clearance opening inviting substantial wear and imposing new lifting force peaks. The advantages of using a coil spring over the elastomeric discs have not fully materialized in view of certain friction action experienced in present coil spring adaptation for a standard shock absorber between the lift bridle and polish rod. This disadvantage is overcome by the present invention so a coil spring shock absorber can provide its benefits without disadvantages of the Pelham effort.
To overcome the disadvantages associated with efforts to use a coil spring in the shock absorber between the reciprocal member of the well pumping mechanism and the polish rod, a shock absorber as shown in Pelham U.S. Pat. No. 6,446,946 has been modified to include a friction reducing centering head secured to the bottom and plate. In practice, a head is used at both the top end plate and the bottom end plate. These centering heads secured to the end plates center the polish rod in the clearance opening in the end plates and hold the polish rod away from the wall surrounding the clearance opening in the end walls. The spring pilot as shown on the lower end wall of Pelham U.S. Pat. No. 6,446,946 is replaced and made as a part of the centering head on the lower end plate so that the clearance opening is not extended for increased frictional engagement as necessary in the prior art. Lateral forces are decreased. The lower, or both end plates of the shock absorber, are provided with the centering mechanism that preferably includes a spring pilot to substantially reduce the friction experienced between the polish rod as it reciprocates in the shock absorber.
In accordance with the invention, a shock absorber for use between the reciprocating member of an oil well pumping mechanism and a polish rod connected to the downwardly extending rod string comprises an upper and lower end plate. Each end plate has a peripheral surface around the axis of the shock absorber to define a polish rod clearance opening between the two end plates. A coil spring is positioned between the end plates in a position concentric with, and surrounding, the reciprocal axis of the polish rod. Attachment members secure the lower plate with the reciprocal member of the well pumping mechanism. In accordance with the invention, a friction reducing centering head is secured to lower or both of the end plates for centering the polish rod as it passes through the clearance opening and for holding the polish rod away from the peripheral wall surrounding and defining the clearance opening. Thus, the clearance openings in the end plates, especially the one in the lower plate, are larger than the polish rod and a centering head engages the reciprocating polish rod with a mechanism to reduce the friction and wear between the reciprocating polish rod and the shock absorber.
In accordance with another aspect of the present invention, the coil spring has an inside diameter and the centering head includes a generally circular spring pilot secured to the end plate. This pilot has a center opening larger than but aligned with the clearance opening of the end plate and an outer diameter slightly smaller than the inner diameter of the coil spring. Thus, the centering head on each of the end plates perform the function of reducing friction, but also performs the function of guiding the coil spring. This guiding action occurs at both the upper end plate and the lower end plate. Consequently, the coil spring is guided in its axial movement during operation of the shock absorber. This is different than Pelham U.S. Pat. No. 6,446,946 wherein the spring aligner or pilot has an opening that is a mere extension of the lower plate clearance opening of the polish rod. This prior design increases the friction between the rod and the shock absorber. Thus, efforts to incorporate the advantages of the coil spring are met with frictional disadvantages. Furthermore, the prior art does not incorporate the advantage of a spring pilot on both reciprocating end plates to guide the main spring.
In accordance with another aspect of the present invention, the centering heads each have at least three guide rolls with rotating outer cylindrical surfaces that engage the polish rod and are mounted on the spring pilot. Thus, the polish rod extends through the large clearance opening in the end plates and is engaged with inwardly extending rotating cylindrical surfaces. These rolls may be elongated or relatively short in axial length. In practice, three circumferentially spaced guide rolls are preferred. The equally spaced guide rolls are rotatably mounted on a hardened pins to reduce the friction caused by engaging and rotating with the polish rod. In accordance with an aspect of the invention, the pins are hardened and the outer surface of the rollers engaging and centering the polish rod have a lower hardness than the hardness of the polish rod. This feature can not be used in the Pelham mechanism. Consequently, even the rolling action between the polish rod and the rolls does not damage or deteriorate the surface of the polish rod. The rolls have a low hardness compared to the polish rod. The pins are hardened steel that have a reduced rotational friction to be combined with a low rotational friction between the polish rod and the outer cylindrical surfaces of the circumferentially spaced rolls.
In the preferred embodiment of the present invention, the upper end plate of the shock absorber has a depending cylindrical body surrounding the coil spring and reciprocally received in an upstanding cylindrical body extending from the lower end plate. In still a further aspect, a second coil spring concentric with the first mentioned coil spring is provided in the shock absorber. In accordance with spring technology, the two springs have opposite coiled directions to prevent interleaving of the coil springs. The springs can have different free lengths so one spring provides the primary constant with other springs providing modified constants based upon the amount of force resisted by the spring combination. Use of two or more coil springs nested together allows correct spring rate at various deflections to optimize the deceleration rate of the shock absorber during the lifting action. The spring pilot associated with the centering head has an outer diameter to prevent the coil spring or inner coil spring from shifting with respect to the center axis of the pumping mechanism. When multiple springs are used, the inner, main spring guides the other spring or springs.
In accordance with another aspect of the present invention, the individual rolls in the centering head are radially adjustable to accommodate different diameter polish rods. Furthermore, one of the rolls is provided with an eccentric mounting mechanism which adjusts the clearance between the rolls and the reciprocating polish rod. In practice, an eccentric roll is provided on the centering head of both the top and bottom end plates. Of course, a single centering head could be used on the bottom end plate of the shock absorber where the most relative movement is created.
In accordance with another aspect of the invention a series of low friction elements, such as rollers are mounted on one cylindrical body of the spring housing to engage the other cylindrical body. This guides the two telescoped bodies as they reciprocate during pumping. This prevents undue wear and any binding between the moving components of the shock absorber.
The primary object of the present invention is the provision of an improved shock absorber for an oil well pump that dampens the peak load during pumping so the sucker rod and polish rod life is extended, as well as the life of the shock absorber. This shock absorber provides reduced wear on the pumping mechanism and the down hole pump.
Another object of the present invention is the provision of an improved shock absorber, as defined above, which shock absorber is used between the reciprocating pumping member of an oil well pumping mechanism and the string of sucker rods to reduce the peak load imposed on the sucker rod string during the pumping action and to increase the life of the shock absorber.
Still a further object of the present invention is the provision of a shock absorber, as defined above, which improved shock absorber utilizes one or more coil springs between the end plates as opposed to elastomeric discs as commonly used in the prior art to obtain the benefits of a coil spring.
A further object of the present invention is the provision of a shock absorber, as defined above, which shock absorber uses a centering head on the upper and lower end plates of the shock absorber whereby the polish rod clears the end plates and is engaged and centered by a separate low friction mechanism to thereby reduce the friction and increase the uniform and consistent performance of the pumping mechanism and the life of the shock absorber and polish rod.
These and other objects and advantages will become apparent from the following description taken together with the accompanying drawings.